Positron emission tomography (PET) is a precise and non-invasive medical imaging technique. In practice, a radiopharmaceutical labelled by a positron-emitting radioisotope, in situ disintegration of which results in the emission of gamma-rays, is injected into the organism of a patient. These gamma-rays are detected and analyzed by an imaging device in order to reconstruct in three dimensions the biodistribution of the injected radioisotope and to obtain its tissue concentration.
Fluorine 18 (T1/2=109.6 min) is the only one of the four light positron-emitting radioisotopes of interest (13N, 11C, 15O, 18F) that has a half-life long enough to allow use outside its site of production.
Among the many radiopharmaceuticals synthesized from the radioisotope of interest, namely fluorine 18, 2-[18F]fluoro-2-deoxy-D-glucose (FDG) is the radio-tracer used most often in positron-emission tomography. It allows the metabolism of glucose in tumours, in cardiology and in various brain pathologies to be analyzed.
The 18F radioisotope is produced by bombarding a target material, which in the present case consists of 18O-enriched water (H218O), with a beam of charged particles, more particularly protons. To produce said radioisotope, it is common practice to use a device comprising a cavity “hollowed out” in a metal part and intended to house the target material used as precursor.
The cavity in which the target material is placed is sealed by a window, called “irradiation window” which is transparent to charged particles of the irradiation beam. Through the interaction of said charged particles with the said target material, a nuclear reaction is generated which leads to the production of the radioisotope of interest.
The beam of charged particles is advantageously accelerated by an accelerator such as a cyclotron.
At the present time, because of an ever increasing demand for radioisotopes, and in particular for the 18F radioisotope, it is requested to increase the yield of the nuclear reaction in order to always produce more radioisotope. This increase in production assumes either to modify the energy of the beam of charged particles (protons), and in this case make use of the dependence of thick target yield on the particle energy, or to modify the intensity of said beam, and in this case the number of accelerated particles striking the target material is modified.
However, the power dissipated by the target material irradiated by the accelerated particle beam limits the intensity and/or the energy of the particle beam that it is used.
This is because the power dissipated by a target material is determined by the energy and the intensity of the particle beam through the following equation (1):P(watts)=E(MeV)×I(μA)  (1)                where:        P=power expressed in watts;        E=energy of the beam expressed in MeV; and        I=intensity of the beam expressed in μA.        
In other words, the power dissipated by a target material is therefore higher the higher the intensity and/or the energy of the particle beam.
It will consequently be understood that the energy and/or the intensity of the beam of accelerated charged particles cannot be increased without rapidly generating, within the cavity of the production device, and especially at the irradiation window, excessive pressures or temperatures liable to damage said window.
Moreover, in the case of 18F radioisotope production, given the particularly high cost of 18O-enriched water, only a small volume of this target material, at the very most a few millilitres, is placed in the cavity. Thus, the problem of dissipating the heat produced by the irradiation of the target material over such a small volume constitutes a major problem to be ovecome. Typically, for a volume of 18O-enriched water of 0.2 to 5 ml, the power to be dissipated is between 900 and 1800 watts for a 18 MeV proton beam with an intensity of 50 to 100 μA and for irradiation times possibly ranging from a few minutes to a few hours.
More generally, given this problem of heat dissipation by the target material, the irradiation intensities for producing radioisotopes are currently limited to 40 μA for an irradiated target material volume of 2 ml. Now, current cyclotrons used in nuclear medicine are, however, theoretically capable of accelerating proton beams with intensities ranging from 80 to 100 μA, or even higher. The possibilities afforded by current cyclotrons are therefore indubitably underexploited.
Solutions have been proposed in the prior art for overcoming the problem of heat dissipation by the target material in the cavity within the radioisotope production device. In particular, it has been proposed to provide means for cooling the target material.
Accordingly document BE-A-1011263 discloses an irradiation cell comprising a cavity sealed by a window, in which cavity the target material is placed, the said cavity being surrounded by a double-walled jacket allowing the circulation of a refrigerant for cooling said target material. Furthermore, it can be contemplated to cool the irradiation window by means of helium.
However, in that device, the target material is static, which gives said device configured in this way a number of drawbacks insofar as the heat dissipation in this configuration is physically limited due to the coefficient of heat exchange of the liquid with its container. Moreover, because of the high temperatures that are reached in the sealed cavity, the entire device must be pressurized. In fact, it is practically impossible to “monitor” the amount of 18F produced in such a device, and the result, in terms of activity and yield, is therefore only known a posteriori.
It has also been proposed (in a publication by Jongen and Morelle, International Symposium “Proceedings of the third workshop on targetry and target chemistry”, Vancouver, June 1989) to use a device in the form of circuit comprising an irradiation cell with a cavity containing a target material and an external heat exchanger in which the said H218O target material is recirculated so as to be cooled. This device, compared with that of the abovementioned prior art, therefore has the advantage of using a target material that can be termed “dynamic” since it is recirculated. Nevertheless, that device and method did not use pressurizing means so that the control of the pressure is a real problem in such a device. Moreover, said device and method were not explained in detail and are in practice prone to major technical implementation difficulties.